Evidence is presented induces lower IgE production in an in vivo transgenic mouse model. In addition, a new method has been developed to induce higher levels of CD23 by inhibiting the metalloprotease involved in CD23 breakdown. The studies proposed will extend these findings. Additional CD23 transgenic animals have been prepared in which the increased CD23 expression is on B cells; these animals will continue to be evaluated for their IgE responses to antigen and parasite regimens, including both extra- and intracellular parasite infections. The cytokine profiles of these animals will be determined with respect to relative Th1 vs. Th2 predominance. Since the initial findings indicate that the membrane form of CD23 is the active agent, the initial proteolytic site on CD23 has been mutated and the mutant CD23 has been shown to bind IgE with the same affinity as the parental form, but is resistant to being cleaved from the surface. Transgenic animals bearing the mutant CD23 will be prepared and analyzed as with the current CD23 transgenics. The mechanism via which the metalloprotease inhibitors influence IgE synthesis will be studied. In vitro experiments will determine if other Ig classes are inhibited at the same level and whether the elevated CD23 induces selective apoptosis of switched cells. In addition, the efficacy of these inhibitors in in vivo models will be determined; this is important with respect to determination of whether these agents have potential as anti-allergic drugs. In the third aim, the influence of the follicular dendritic cells (FDC) on IgE production will be determined by using a modification of the in vitro model used in Aim 2. FDCs from CD23 transgenic animals will be isolated and cultured with normal B cells and the IgE levels obtained will be compared to that obtained with B cells cultured with FDCs from normal controls. In the fourth aim, an analogous co-culture model using human CD23 expressing CHO cells as well as a newly developed sCD23 with full IgE binding capacity will be evaluated using the in vitro human PBL model again examining the influence of high CD23 on B cell proliferation and IgE production with the eventual aim of developing a system for control of human IgE production.